Semiconductor nanoparticles whose particle sizes are 10 nm or less are located in the transition region between bulk semiconductor crystals and molecules. Their physicochemical properties are therefore different from both bulk semiconductor crystals and molecules. In this region, due to the quantum-size effect, the energy gap of semiconductor nanoparticles increases as their particle sizes decrease. In addition, the degeneration of the energy band that is observed in bulk semiconductors is removed and the orbits are dispersed. As a result, the lower-end of the conduction band is shifted to the negative side and the upper-end of the valence band is shifted to the positive side.
Semiconductor nanoparticles of CdS can be easily prepared by dissolving equimolar amounts of precursors of Cd and S. This is also true for the manufacturing of, for example, CdSe, ZnS, ZnSe, HgS, HgSe, PbS, or PbSe.
Semiconductor nanoparticles have drawn attention since they emit strong fluorescences whose full widths at half maximum are narrow. Thus, various fluorescent colors can be created, and numerous future applications, for example, detection of genes or proteins, optical devices, or the use thereof in the medical field, can be expected. However, the semiconductor nanoparticles obtained only by mixing the precursors with each other as described above have a wide distribution of particle sizes and therefore cannot provide the full advantage of the properties of semiconductor nanoparticles. Accordingly, attempts have been made to attain a monodisperse distribution by using chemical techniques to precisely separate and extract only the semiconductor nanoparticles of a specific particle size from semiconductor nanoparticles having a wide distribution of particle sizes immediately after preparation. The attempts to attain a monodispersed distribution of particle sizes that have been reported so far include: separation by electrophoresis that utilizes variation in the surface charge of nanoparticles depending on their particle sizes; exclusion chromatography that utilizes differences in retention time due to different particle sizes; and size-selective precipitation that utilizes differences in dispersibility in an organic solvent due to differences in particle sizes.
A method was described above wherein the nanoparticles, which were prepared by mixing the precursors with each other, were separated depending on their particle sizes. Also reported is size-selective photoetching that attains a monodispersed distribution of particle sizes by utilizing the oxidative dissolution of a metal chalcogenide semiconductor in the presence of dissolved oxygen when irradiated with light.
There is also a method wherein a monodispersed distribution of particle sizes is attained through regulation at the phase of mixing the precursors with each other. A representative example thereof is the reversed micelle method. In the reversed micelle method, amphiphilic molecules such as diisooctyl sodium sulfosuccinate are mixed with water in an organic solvent such as heptane to form a reversed micelle therein, and precursors are allowed to react with each other only in an aqueous phase in the reversed micelle. The size of the reversed micelle is determined according to the quantitative ratio of the amphiphilic molecules to water, and its size can be relatively uniformly regulated. The sizes of prepared semiconductor nanoparticles depend on the size of the reversed micelle. Thus, semiconductor nanoparticles with relatively homogenous particle sizes can be prepared.
An example of a representative method for preparing semiconductor nanoparticles was provided above, although suitable preparation methods vary depending on materials constituting semiconductor nanoparticles. For example, CdSe is generally prepared in a coordinated solvent at a relatively high temperature. In contrast, CdS can be prepared by a method equivalent to the aforementioned method. However, it is more suitable to prepare CdS in an aqueous solution by size-selective photoetching.
As mentioned above, the present inventors have attempted to develop a method of size-selective photoetching wherein semiconductor nanoparticles are prepared in an aqueous solution. In accordance therewith, they have examined materials for surface modification in an aqueous solution. Accordingly, a method for preparing semiconductor nanoparticles is selected depending on materials constituting the semiconductor nanoparticles. When a preparation method that is carried out in an aqueous solution is not suitable, various techniques that have been developed for use in an aqueous solution could not be applied to the resulting semiconductor nanoparticles.
More specifically, in order to apply the results of the research conducted by the present inventors to various materials for semiconductor nanoparticles, nanoparticles, which were prepared outside of an aqueous solution, had to be converted into conditions equivalent to semiconductor nanoparticles, which were prepared in an aqueous solution.
The present invention is aimed at finding a solution to the above problem. The first aspect of the present invention relates to a method for converting materials for modifying surfaces of semiconductor nanoparticles wherein semiconductor nanoparticles are modified with oil-soluble materials for surface modification, the oil-soluble materials for surface modification are converted into water-soluble materials for surface modification at the interface between an organic solvent and water, and the semiconductor nanoparticles are shifted from an organic phase to an aqueous phase by the aforementioned conversion.
Semiconductor nanoparticles that are used in the present invention are not particularly limited, and conventional nanoparticles represented by a general formula MX are used. In this formula, M is a metal atom and selected from Zn, Cd, Hg, In, Ga, Ti, W, Pb, and the like, and X is selected from O, S, Se, Te, P, As, N, and the like. Specific examples are ZnO, ZnS, ZnSe, ZnTe, CdO, CdS, CdSe, CdTe, HgS, HgSe, HgTe, InP, InAs, GaN, GaP, GaAs, TiO2, WO3, PbS, and PbSe.
Also, multi-layer semiconductor nanoparticles comprising a core portion and a cover layer can be preferably used. The multi-layer semiconductor nanoparticles that are disclosed in WO99/26299 can be used. A specific example of preferable multi-layer semiconductor nanoparticles comprises a core selected from the group consisting of Cd and X (wherein X represents S, Se, Te, or a mixture thereof) and a cover layer of ZnY (wherein Y represents S or Se and is uniformly laminated on the core). In addition, materials for the core and materials for the cover layer uniformly laminated on the surface of the core are selected from ZnO, ZnS, ZnSe, ZnTe, CdO, CdS, CdSe, CdTe, HgS, HgSe, HgTe, InP, InAs, GaN, GaP, GaAs, TiO2, WO3, PbS, PbSe, or the like. An example thereof is the case of multi-layer semiconductor nanoparticles in which the material for the core is CdS and the material for the cover layer uniformly laminated on the surface of the core is ZnS.
Oil-soluble materials for surface modification are bound to or located on surfaces of semiconductor nanoparticles to dissolve or disperse the semiconductor nanoparticles in an organic solvent. Specifically, preferable examples thereof are tri-n-octylphosphine (TOP) and/or tri-n-octylphosphine oxide (TOPO).
Water-soluble materials for surface modification are bound to or located on surfaces of semiconductor nanoparticles to dissolve or disperse the semiconductor nanoparticles in an aqueous medium. Specifically, a preferable example thereof is a thiol compound such as mercaptopropionic acid (MPA).
The second aspect of the present invention relates to a method for purifying semiconductor nanoparticles wherein semiconductor nanoparticles, the surfaces of which have been modified with the water-soluble materials for surface modification obtained by the first aspect of the present invention, are subjected to size-selective photoetching, thereby regulating particle sizes of the semiconductor nanoparticles, and monodispersing them.
FIG. 1 is a pattern diagram showing embodiments according to the first and the second aspects of the present invention, wherein semiconductor nanoparticles of cadmium sulfide are dissolved in an organic solvent and stabilized with tri-n-octylphosphine (TOP). The TOP therein is converted with the aid of hydrophilic mercaptopropionic acid (MPA) to shift it into an aqueous solution. Subsequently, the semiconductor nanoparticles of cadmium sulfide, which were stabilized with MPA, are subjected to size-selective photoetching and modification with sodium hexametaphosphate (HMP).
Size-selective photoetching takes advantage of the fact that the energy gap of a semiconductor nanoparticle increases due to the quantum-size effect as the particle size thereof decreases and that a metal chalcogenide semiconductor is oxidatively dissolved in the presence of dissolved oxygen when irradiated with light. In this method, the semiconductor nanoparticles having a wide distribution of particle sizes are irradiated with monochromatic light of a wavelength shorter than the wavelength of the semiconductor nanoparticle's absorption edge. This causes only the semiconductor nanoparticles of larger particle sizes to be selectively photoexcited and dissolved, thus sorting the semiconductor nanoparticles into smaller particle sizes. Due to the quantum-size effect, the physicochemical properties of semiconductor nanoparticles depend on their particle sizes. Accordingly, the physical properties of these semiconductor nanoparticles in this state are averaged out and their traits cannot be fully manifested. Thus, a chemical technique, i.e., size-selective photoetching, is utilized to precisely separate and extract only the semiconductor nanoparticles of a specific particle size from semiconductor nanoparticles having a wide distribution of particle sizes immediately after preparation in order to attain monodispersed distributions.
The semiconductor nanoparticles of the present invention have significant fluorescence properties. In particular, the fluorescence properties are strongly exhibited when the semiconductor nanoparticles have a monodispersed distribution of particle sizes. More specifically, the particle sizes of the semiconductor nanoparticles are preferably monodispersed, so that deviations are less than 10% rms in diameter.
The fluorescence emitted by the semiconductor nanoparticles of the present invention has a sharp peak of fluorescence intensity. The semiconductor nanoparticles can also emit fluorescence in a narrow spectrum range of 60 nm or less in terms of the full width at half maximum (FWHM). It is preferably 40 nm or less, and more preferably 30 nm or less in terms of the full width at half maximum (FWHM).
The third aspect of the present invention relates to a method for purifying semiconductor nanoparticles wherein semiconductor nanoparticles, the surfaces of which have been modified with the water-soluble materials for surface modification obtained by the first aspect of the present invention, are subjected to size-selective photoetching, and the dissolution caused thereby is utilized to peel the surfaces of the semiconductor nanoparticles, thereby converting the materials for surface modification.
As described above, surfaces of semiconductor nanoparticles were stabilized by modifying them with, for example, a thiol compound, the modified surfaces were again subjected to size-selective photoetching to etch surfaces only, and substances other than semiconductor nanoparticles were removed by ultrafiltration. This enabled nanoparticles, which were prepared outside of an aqueous solution, to be converted into conditions equivalent to semiconductor nanoparticles prepared in an aqueous solution.